Single nozzle free-vortex aerodynamic window

ABSTRACT

A gas laser emits an output laser beam from a low pressure region to a region having a higher pressure. This is done through an opening in the side of a laser device. Under normal circumstances, the provision of such an opening would incur a flow of air from the high pressure side, externally of the laser device, to the low pressure side within the device. To prevent this, an aerodynamic window is placed in a passageway leading from said opening in the laser device through which the laser beam passes. A gas flow is passed across said passageway which will provide a flow which represents the flow of a segment of a free-vortex flow field having a pressure differential across the segment which is equal to that between the low and high pressure regions. A single nozzle, specifically contoured to produce a segment of a free-vortex flow field directs the gas into the passageway and an opening collects the flow onto the other side of said passageway. A method of constructing such a nozzle is set forth.

BACKGROUND OF THE INVENTION

This invention relates to aerodynamic windows in general and is shown incombination with a gas laser. In lasers of low power, windows withphysical walls made of materials which transmit the laser wavelengthhave been used, but subject window is for use when the laser beam willdistort or disintegrate physical window materials. Other aerodynamicwindows of this type are set forth in U.S. Pat. Nos. 3,604,789,3,617,928, 3,654,569, 3,873,939 and application Ser. No. 437,114 nowU.S. Pat. No. 3,907,409.

SUMMARY OF THE INVENTION

A primary object of this invention is to provide an aerodynamic windowwhich would permit passage of a laser beam with no physical obstructionsyet prevent or minimize flow through said window between two regions ofdifferent pressures.

In accordance with the present invention, flow of an aerodynamic windowbetween two regions of different pressure produces a flow field whichrepresents a segment of a free vortex by the use of an appropriatelycontoured free-vortex nozzle.

An object of this invention is to reduce the gas supply mass flow to besupplied to an aerodynamic window to create the gas jet which is used toisolate the low pressure region from the high pressure region.

Another object of this invention is to provide a method of constructinga free-vortex nozzle to obtain a section of supersonic free-vortex flowtherefrom.

A further object of this invention is to provide a method ofconstructing a free-vortex nozzle by determining the velocitydistribution necessary to form a section of a supersonic free-vortexflow, then constructing the internal contour of a free-vortex nozzle byfirst constructing the exit kernel of a nozzle flow field using theknown method of characteristics and then constructing the inner andouter nozzle contours by patching segments of the contours and theircontiguous flow fields to the exit kernel carrying the constructionbackward through the nozzle by the method of characteristics to itsthroat.

It is a further object of this invention to provide an aerodynamicwindow which will reduce laser beam quality degradation to an acceptablelevel.

Another object of this invention is to reduce changes in beam directionand jitter, which is fluctuation of beam direction.

It is another object of this invention to provide a compact aerodynamicwindow for small or focused beams.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view of the aerodynamic window as shown in relation to alaser device;

FIG. 2 is an enlarged view of a free-vortex nozzle of the aerodynamicwindow;

FIG. 3a is a diagram showing an end view taken through free-vortex flowfield;

FIG. 3b is a diagram of a segment of a free-vortex flow field;

FIG. 3c is a graph showing a representative curve of free-vortexvelocity distribution giving the variation of velocity ratio with radiusratio;

FIG. 3d is a graph showing a representative curve of free-vortexpressure distribution obtained using the velocity distribution of FIG.3c and the isentropic flow relationships;

FIG. 4a is a view showing the exit kernel (I-II; II-III and III-I) ofthe nozzle flow field which is the region bounded by the exit plane, theright-running characteristic passing through f, and the left-runningcharacteristic passing through a;

FIG. 4b is a view showing the exit kernel and a second kernel (II-III;III-IV and IV-II) of the nozzle flow field which is the region boundedby the right-running characteristic passing through f, the extension ofthe left-running characteristic passing through a, and the outer contourof the nozzle flow having the radius R_(U) ;

FIG. 4c is a view showing the exit kernel, second kernel and a thirdkernel (I-III-IV; IV-V and I-V) of the nozzle flow field which is theregion bounded by the left-running characteristic passing through a, theinner contour of the nozzle flow having the radius R_(L), and theright-running characteristic (IV-V) passing through the intersection ofthe outer contour and the left-running characteristic passing through a;

FIG. 4d is a view showing the exit kernel, second kernel, third kerneland a fourth kernel and conventional uniform flow nozzle; the fourthkernel (IV-V; V-VI and VI-IV) of the nozzle flow field is a so-calledsimple flow region constructed by the rearward projection of theleft-running characteristics which intersect the right-runningcharacteristic IV-V. As required for a simple flow region, the rearwardprojection of these left-running characteristics is terminated whereconservation of mass is satisfied and no reflection of right-runningcharacteristics occurs. The termination of the characteristics definesthe portion of the nozzle wall VI-IV. The conventional uniform flownozzle is patched to the simple flow region along the left-runningcharacteristic VI-V. This uniform nozzle represents one-half of theusual supersonic wind tunnel or perfect nozzle in that one wall(terminating at VI) is flat and is substituted for the line of symmetryof the wind tunnel nozzle.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A gas dynamic laser 1 comprises a nozzle means 3, a laser cavity 5 and adiffuser section not shown. A similar nozzle means is disclosed in U.S.Pat. No. 3,602,432. More information on a laser construction can befound in the patents referred to above and much other prior art. Thelaser beam X formed in the cavity 5 is directed through an opening 9 inthe side of the laser. A mirror 8 is part of the optical system whichdirects th beam X through the opening 9. A passageway 11 extendsdownwardly from and around the opening 9 through which the laser beam Xpasses from the cavity. A centrifugal aerodynamic window 2 is providedwhich is fixedly connected to the passageway 11 to form an aerodynamicboundary across said passageway so that the difference in pressurebetween that in the cavity 5 of the laser 1 and the pressure externallyof the laser is prevented from equalizing.

The aerodynamic window 2 includes a conduit 13 having an opening 6placed at the end of the passageway 11 through which a gas underpressure is directed across the passageway. This gas flow is received inan opening 7 on the opposite side of the passageway. A free-vortexsupply nozzle 14 is located in conduit 13 adjacent the opening 6 anddirects its flow therethrough into passageway 11 so as to enter opening7. A pump 12 connected to conduit 13 supplies high pressure air or othergas, to the nozzle through said conduit 13.

Opening 7 provides an entrance to a diffuser 16 which extends from theopening 7 to the point A. This diffuser 16 serves to increase the staticpressure of the flow either to permit the flow to be exhausted to theatmosphere in an open loop system or reducing the requirements of thepump 12 in a closed system such as that shown in FIG. 1. The diffuser 16empties into a duct 18 which in turn directs the flow back into an inletopening in the pump 12. This provides a closed loop system which willconserve aerodynamic window gas.

The aerodynamic window 2 uses its free-vortex nozzle 14 to produce aflow field across the passageway 11 which will cover the entire openingand which represents a segment of a free-vortex flow field. Thecharacteristics of a free-vortex flow field can be described with theaid of FIG. 3a. FIG. 3a shows a free-vortex flow field wherein thestreamlines are concentric circles and the velocity distribution is suchthat the product of velocity and radius is a constant. In FIG. 3a thenon-dimensional velocity is shown as the ratio of the local velocity tothe well-known critical sound speed and the radius R₀ is that radiuswhere the value of v/a_(*) = 1.0 so that the free-vortex flow fieldcondition is defined by the requirement that the product of (v/a_(*))and (r/R₀) is a constant. The resulting velocity distribution given inFIG. 3c, can be used to determine the ratio of static-to-total pressurethroughout the free-vortex flow field (FIG. 3d) from the equation##EQU1## for compressible isentropic flow (γ is the ratio of specificheats).

FIG. 3b illustrates a segment of the free-vortex flow field and isbounded by arcs of radii R₁ and R₂ and is included within an angularsegment. Δθ, of the 360°C flow field of FIG. 3a. This segment of thefree-vortex flow field is sized to span the passageway 11 of theaerodynamic window 2 with D equal to the width of the passageway 11,while having a pressure on one side equal to the pressure in the lasercavity and having the pressure on the other side equal to the pressureof the atmosphere.

The conditions defining the free-vortex flow field of which a segment isto be approximated are determined to establish appropriate size.Determination of the free-vortex flow field conditions begins with theselection of the properties of the supply gas. The gas composition,supply total temperature, T_(T).sbsb.a.sbsb.w, and supply totalpressure, P_(T).sbsb.a.sbsb.w, are chosen to be compatible with thesystem for which the window is being constructed. For a laser system thesupply gas must be transparent to the laser radiation, e.g. dry air ornitrogen for a gas dynamic laser. The ratios of laser cavity pressure tosupply total pressure, P_(L) P_(T).sbsb.a.sbsb.w, and ambient pressureto supply total pressure, P_(AMB) /P_(T).sbsb.a.sbsb.w, are formeddefining the static-to-total pressure ratios at the respective radii R₁and R₂ bounding a segment of the free-vortex flow field. These pressureratios define the values of R₁ /R₀ and R₂ R₀ as indicated in FIG. 3d,and in turn fix the velocity ratios v/a_(*) at R₁ and R₂ through thefree-vortex flow field conditions of FIG. 3c.

The value of R₁ can be calculated trigonometrically from FIG. 3b interms of the size, D, of the passageway 11 and the flow turning angleΔθ. While the value of Δθ may be chosen arbitrarily, it is desirable tomaximize Δθ since the mass flow required by the window varies inverselywith the sine of Δθ/2. The maximum value of Δθ will be influenced bylosses in momentum due to turbulent dissipation of the aerodynamicwindow flow and must be determined experimentally. Values of Δθ between60° and 70° have been used successfully. With v/a_(*) known at R₁ andR₂, and R₁ found trigonometrically, the velocity-radius relationship fora free-vortex flow field (FIG. 3a) are used to find R₂.

The aerodynamic window 2 uses a free-vortex supply nozzle 14 to producethe desired free-vortex flow segment across R₁ -R₂. The walls of thisnozzle are contoured to obtain the desired free-vortex velocity andpressure distributions at the opening 6 located at the end of passageway11. The nozzle construction procedure is set forth here: Referring toFIG. 4a, as characteristic of a free vortex, the product of velocity andradius at each of the points a through f lying in the exit plane is aconstant and the velocity direction is perpendicular to the nozzle exitplane. As described above, the value of R₁ is fixed by the aperture, orpassageway 11, size and the flow turning angle, while the velocityratios v/a_(*) at a and f are determined from the gas supply conditions,including composition of the supply gas to be used and its total(stagnation) temperature and pressure, and the laser cavity and ambientpressures.

The flow field within the region bounded by I-II, II-III and I-III inFIG. 4a is found at the intersections of the left-running andright-running characteristics which pass through the points a through fof the exit plane. These characteristics are uniquely defined by theflow conditions at the points a through f. The boundaries of the exitkernel so formed are determined from the intersections of right-runningcharacteristics passing through the points a through f with theleft-running characteristic passing through a (line I-III) and theintersections of the left-running characteristics passing through thepoints a through f with the right-running characteristic passing throughf (line II-III). Note that, while only six points within the nozzle exitplane are shown in FIG. 4a, a finer mesh containing 25 points wasemployed in the computer code developed for designing the free-vortexsupply nozzle.

The next step in forming the nozzle is the selection of the inner andouter wall contours passing through points a and f respectively. At thisstage of the construction, the choice of a wall shape is arbitrarilyprovided the tangent to the contour is perpendicular to the nozzle exitplane at points a and f. This tangency requirement is satisfied if thewall contours are circular arcs whose centers of curvature are locatedalong the nozzle exit plane.

In the example described, wall contours comprised of circular arcs wereused. The centers of curvature O' and O", of these arcs forming outerand inner contours respectively, are located along the extension of theside of the angle Δθ of FIG. 1 which passes through the nozzle exitplane. Values of the radii, R_(U) and R_(L), of the outer and innercontours having the centers O' and O" are selected arbitrarily and theflow field within the kernel bounded by the contour is computed usingthe known method of characteristics. Efficient nozzle shapes arerestricted to those which do not cause a decrease in Mach number in theflow direction along the inner and outer contours. In order that theMach number not decrease in the flow direction, R_(U) must be less thanor equal to R₂ and R_(L) must be greater than or equal to R₁.

For an outer wall radius, R_(U), centered at O' the flow conditionswithin the kernel defined by II-III; III-IV and IV-II in FIG. 4b arecomputed using the flow conditions existing along the right-runningcharacteristic of the exit kernel which passes through point f (II-III).In this example, R_(U) is taken equal to R₂ and O' is coincident with O.The point IV on the outer wall (terminating the arc of radius R_(U)) islocated during the construction at the intersection of the wall and theleft-running characteristic passing through a.

Similarly, by prescribing the radius of curvature of the inner wall,R_(L), centered at O" and using the flow conditions along theleft-running characteristic I-III-IV, the flow field within the kernelbounded by I-III-IV, IV-V and I-V is computed as indicated in FIG. 4c.Point V on the inner contour (terminating the arc of radius R_(L)) islocated during the construction at the intersection of the wall and theright-running characteristic passing through IV. In choosing the outerand inner wall contours described by R_(U) and R_(L), care must be takento insure that the intersection of like characteristics, (e.g., theintersection of two left-running characteristics) does not occur withinthe flow field.

The circular arcs of the inner and outer contours may be extendedfurther back into the nozzle beyond V and IV if intersection of likecharacteristics does not occur or, alternatively, some other radius ofcurvature or contour shape may be selected. The solution to the flowconditions would proceed as described above. In order to arrive at auniform nozzle throat velocity profile, however, the solution musteventually degenerate to a simple wave region within which either left-or right-running characteristics (but not both) exist. This simple flowregion is constructed from the rearward projection of the left-runningcharacteristics passing through IV-V and the wall is defined at thetermination of these characteristics. These characteristics must beterminated so that the nozzle mass flow is conserved. This simple waveregion is bounded by IV-V, V-VI and VI-IV as indicated in FIG. 4d. Sincethe flow conditions along V-VI are everywhere constant, line V-VI can beused as the terminating characteristic of a conventionalconverging-diverging nozzle designed to produce uniform flow atconditions identical to those existing along line V-VI. This patching ofthe nozzle portion bounded by I-II-IV-VI-V-I with a uniform flow nozzlecompletes the design of the free-vortex supply nozzle. The uniformnozzle consists of one-half of the conventional wind tunnel nozzleformed by placing a flat wall along the line of symmetry of the windtunnel nozzle.

The nozzle shape determined above was evolved ignoring viscous effects(boundary layer growth). The next step in completing the construction isthe calculation of the boundary layer growth along the inner and outercontours. The contours are then corrected to account for the boundarylayer. If the boundary layers are too thick to enable the application ofstandard correction procedures, then the nozzle shape should beredesigned by choosing different values for the wall radii and repeatingthe above method.

While a nozzle contour having sections comprised of singular circulararcs on both inner and outer contours was used in the example discussedabove, other combinations having more than one circular arc radius oneach contour or even shapes other than circular arcs could be employedfor the nozzle contours provided that such other shapes result in afree-vortex flow field at the nozzle exit. The design procedureemploying the method of characteristics would be similar to that setforth above.

In an aerodynamic window constructed using the above procedure, thepassageway 11 was 3.8 cm in width, the supply total pressure was 10 atm,and a flow turning angle Δθ of 60° was employed. The exit plane of thenozzle 14 was positioned along an extension of a side of the angle Δθ.The radius R₁ was 3.8 cm and the radius R₂ was 4.76 cm. An aerodynamicwindow of this type was built to seal a cavity at a pressure of 1/15 atmfrom the atmosphere. The nozzle had an outer wall radius R_(U) of 3.8 cmand an inner wall radius R_(L) of 10.02 cm with simple flow regionsalong both outer and inner walls patched to a uniform nozzle having avelocity ratio of v/a_(*) = 1.493. The velocity ratios at the exit ofthe nozzle corresponded to the flow conditions dictated by the choise ofthe 10 atm supply pressure and the pressures of the laser cavity andatmospheric surroundings. The diffuser 16 of FIG. 1 has a longintermediate section L which was made approximately ten times the widthof the throat e to facilitate exhaust of the aerodynamic window flow.

We claim:
 1. In combination in a gas laser device, a lasing region oflow pressure, an outlet for an output laser beam in said laser deviceforming an exit to a second region of a different pressure, an outletpassageway connected to said laser device and extending away therefromaround said outlet, means comprising a single free-vortex nozzle forproviding an arcuate gas flow across said passageway which representsthe flow of a segment of a free-vortex flow field having a pressuredifferential which is equal to that between the lasing region and thesecond region.
 2. A combination as set forth in claim 1 wherein saidnozzle has its exit plane along a radius which forms one side of thesegment of a free-vortex flow field and passes through the center ofcurvature of the segment of a free-vortex flow field.
 3. In combinationin a system having a region of low pressure, an opening in said regionforming an exit, an outlet passageway connected to said system andextending away therefrom around said opening, and an aerodynamic windowassembly permitting passage therethrough to a region of higher pressure,said aerodynamic window assembly comprising a single free-vortex supplynozzle for providing an arcuate gas flow across said passageway whichrepresents the flow of a segment of a free-vortex flow field having apressure differential which is equal to that between the lasing regionand the second region.